1. Field of the Invention
The invention relates to an image data decoding method of image vertical blanking interval (VBI) and a device to perform the method and, more particularly, to an image data decoding method for high-frequency teletext image data of the image vertical blanking interval.
2. Description of the Related Art
When processing an image display, VBI (vertical blanking interval) and HBI (horizontal interval) signals are normally reserved without showing. Conventional VBI data types are mainly three types, TeleText, Closed Caption (CC), and Wide Screen Signal (WSS). With reference to FIG. 5, every data line of the teletext-type VBI data includes a 2-byte run-in clock signal and a 43-byte data string. With reference to FIG. 6, every data line of the CC-type or WSS-type data includes a 1-byte run-in clock signal and a 2-byte data string.
When processing the VBI data, the VIII data has to be decoded in advance. A decoding method for the teletext-type data is more complicated, because the run-in clock signal of the teletext includes a higher frequency than that of the CC or WSS, and, also, the data volume of the teletext is larger than that of the CC or WSS.
With reference to FIG. 7, a teletext decoder uses a closed phase locked loop (PLL) circuit to lock the run-in clock signal. The closed phase locked loop circuit separates the teletext data lines through a run-in clock signal separator to accurately read the data of the teletext data lines.
Although the described closed phase locked loop circuit is beneficial to adjust different phases of the data in the teletext data lines, the run-in clock signal of the teletext data lines is only a small part of the data, which does not occur frequently. Moreover, the closed phase locked loop needs to take quite some time to be able to accurately lock the run-in clock signal. Since digital data of the teletext data lines is not continuous, a phase error when decoding the teletext data lines can easily occur by using the closed phase locked loop method to track data over a long term.
With reference to FIG. 8, U.S. Pat. No. 5,657,088 entitled “System and method for extracting caption teletext information from a video signal” uses a threshold value to determine logical values of the data. When the input digital image data exceeds a setup turning point, the logical value is determined to be 1 or 0. On the other hand, when the input digital image data is lower than the setup turning point, the logical value is determined to be 0 or 1.
Further, a delay time can be set for the current input digital image data, so the digital input data can be determined to be beyond or below a turning point to complete decoding the digital image data. The decoding method in this patent is rather simple and can save space in a layout area of image processing chips. However, the turning point is not easy to change, which tends to cause an erroneous decoding result when the image processing chips process image signals in an inferior signal environment.